Phase change materials (PCM) are capable of storing heat energy in the form of latent heat. Such materials undergo a phase transition when heat is supplied or removed, e.g., a transition from the solid to the liquid phase (melting) or from the liquid to the solid phase (solidification) or a transition between a low-temperature and high-temperature modification or between a hydrated and a de-hydrated modification or between different liquid modifications. If heat is supplied to or removed from a phase change material, on reaching the phase transition point the temperature remains constant until the material is completely transformed. The heat supplied or released during the phase transition, which causes no temperature change in the material, is known as latent heat.
Phase change materials can be used to store latent heat in a latent heat storage device. The device is charged by transfer of heat (thermal energy) from a medium which releases heat across an interface into the phase change material where the heat is stored, and discharged by transfer of heat from the phase change material across the interface to a medium which is heated. The medium from/to which heat is transferred can be, for example, water or steam, air, helium or nitrogen. The heat exchange interface can consist, for example, of the walls of heat exchange tubes passed through by a medium from/to which heat is transferred, or of heat exchanger plates over which a medium flows from/to which heat is transferred.
Unfortunately, the thermal conductivity of most phase change materials is rather low (in the range between 0.2 W/m*K and 1.0 W/m*K). As a consequence, the charging and discharging of a latent heat storage device are relatively slow processes. Therefore, in order to reduce the distance over which heat must be transferred within the bulk of the phase change material, the thickness of the layer of phase change material adjacent to the heat exchange interface tends to be minimized. For example, when heat is transferred from or to the phase change material via heat exchange tubes, a small distance between the tubes is necessary in order to reduce the thickness of the PCM layer through which heat has to be transferred from/to the tubes. A representative distance between tubes having a tube diameter of 10 to 50 millimeters (mm) may be on the order of 50 to 80 mm. Due to the requirement of a small distance between the tubes, the number of tubes needed for a heat storage device of a given capacity becomes quite large. This is very disadvantageous for technical and economical reasons, because the large number of tubes makes the device difficult to manufacture and handle, and expensive.
The problem of the low thermal conductivity of phase change materials can be overcome by providing a latent heat storage composite wherein the phase change material is combined with an auxiliary component with high thermal conductivity, e.g., graphite.
U.S. Publication No. 2004-084658 discloses a heat storage device using a latent heat storage composite comprising a phase change material and a volume fraction of 5 to 40% of expanded graphite particles. A bulk bed of the latent heat storage composite is introduced into a latent heat storage vessel interspersed with heat exchanger profiles.
Throughout the latent heat storage composite, the graphite particles are distributed substantially homogeneously. Hence there is no prevailing directional orientation of the heat conduction. However, a preferred conduction of heat in the direction to/from the interface where the heat exchange occurs is desired in order to enhance the charge or discharge of the latent heat storage device.